Pulmonary hypertension (PH) is characterized by an abnormally high blood pressure in the lung vasculature. It is a progressive, lethal disease that leads to heart failure and can occur in the pulmonary artery, pulmonary vein, or pulmonary capillaries. Symptomatically patients experience shortness of breath, dizziness, fainting, and other symptoms, all of which are made worse by exertion. There are multiple causes, and can be of unknown origin, idiopathic, and can lead to hypertension in other systems, for example, portopulmonary hypertension in which patients have both portal and pulmonary hypertension.
Pulmonary hypertension has been classified into five groups by the World Health Organization (WHO). Group I is called pulmonary arterial hypertension (PAH), and includes PAH that has no known cause (idiopathic), inherited PAH (i.e., familial PAH or FPAH), PAH that is caused by drugs or toxins, and PAH caused by conditions such as connective tissue diseases, HIV infection, liver disease, and congenital heart disease. Group II pulmonary hypertension is characterized as pulmonary hypertension associated with left heart disease. Group III pulmonary hypertension is characterized as PH associated with lung diseases, such as chronic obstructive pulmonary disease and interstitial lung diseases, as well as PH associated with sleep-related breathing disorders (e.g., sleep apnea). Group IV PH is PH due to chronic thrombotic and/or embolic disease, e.g., PH caused by blood clots in the lungs or blood clotting disorders. Group V includes PH caused by other disorders or conditions, e.g., blood disorders (e.g., polycythemia vera, essential thrombocythemia), systemic disorders (e.g., sarcoidosis, vasculitis), metabolic disorders (e.g., thyroid disease, glycogen storage disease).
Pulmonary arterial hypertension (PAH) afflicts approximately 200,000 people globally with approximately 30,000-40,000 of those patients in the United States. PAH patients experience constriction of pulmonary arteries which leads to high pulmonary arterial pressures, making it difficult for the heart to pump blood to the lungs. Patients suffer from shortness of breath and fatigue which often severely limits the ability to perform physical activity.
The New York Heart Association (NYHA) has categorized PAH patients into four functional classes, used to rate the severity of the disease. Class I PAH patients as categorized by the NYHA, do not have a limitation of physical activity, as ordinary physical activity does not cause undue dyspnoea or fatigue, chest pain, or near syncope. Treatment is not needed for class I PAH patients. Class II PAH patients as categorized by the NYHA have a slight limitation on physical activity. These patients are comfortable at rest, but ordinary physical activity causes undue dyspnoea or fatigue, chest pain or near syncope. Class III PAH patients as categorized by the NYHA have a marked limitation of physical activity. Although comfortable at rest, class III PAH patients experience undue dyspnoea or fatigue, chest pain or near syncope as a result of less than ordinary physical activity. Class IV PAH patients as categorized by the NYHA are unable to carry out any physical activity without symptoms. Class IV PAH patients might experience dyspnoea and/or fatigue at rest, and discomfort is increased by any physical activity. Signs of right heart failure are often manifested by class IV PAH patients.
Patients with PAH are treated with an endothelin receptor antagonist (ERA), phosphodiesterase type 5 (PDE-5) inhibitor, a guanylate cyclase stimulator, a prostanoid (e.g., prostacyclin), or a combination thereof. ERAs include abrisentan (Letairis®), sitaxentan, bosentan (Tracleer®), and macitentan (Opsumit®). PDE-5 inhibitors indicated for the treatment of PAH include sildenafil (Revatio®), tadalafil (Adcirca®). Prostanoids indicated for the treatment of PAH include iloprost, epoprosentol and treprostinil (Remodulin®, Tyvaso®). The one approved guanylate cyclase stimulator is riociguat (Adempas®). Additionally, patients are often treated with combinations of the aforementioned compounds.
Portopulmonary hypertension is defined by the coexistence of portal and pulmonary hypertension, and is a serious complication of liver disease. The diagnosis of portopulmonary hypertension is based on hemodynamic criteria: (1) portal hypertension and/or liver disease (clinical diagnosis-ascites/varices/splenomegaly), (2) mean pulmonary artery pressure>25 mmHg at rest, (3) pulmonary vascular resistance>240 dynes s/cm5, (4) pulmonary artery occlusion pressure<15 mmHg or transpulmonary gradient>12 mmHg. PPH is a serious complication of liver disease, and is present in 0.25 to 4% of patients suffering from cirrhosis. Today, PPH is comorbid in 4-6% of those referred for a liver transplant.
Despite there being treatments for PAH and PPH, the current prostacyclin therapies are associated with severe toxicity and tolerability issues, as well as the requirement for inconvenient dosing schedules. The present invention overcomes addresses these factors by providing compounds and treatment schedules that provide for less toxicity, better tolerability and more convenient dosing schedules.